1. Field of the Invention
The present invention is directed to an anaesthetic machine for delivering anaesthetic to the airways of a patient, the anaesthetic machine being of the type having a gas flow generator with an inlet and an outlet, and a first system of flow paths connected between this inlet and outlet and a second system of flow paths also connected between this inlet and outlet.
2. Description of the Prior Art
Anaesthetic machines are usually based on a circular (loop) system in which breathing gas is returned to the patient after exhalation. The main reason for this is to minimize the consumption of expensive anaesthetic. Breathing gas is purified in a carbon dioxide absorber. Breathing can either be spontaneous or maintained with gas delivered to the patient in pulses. The pulses are usually generated by a bellows or piston. This kind of anaesthetic machine is usually referred to as a low-flow system. This is because a volume of gas, less than the total volume of gas in the circle system, usually passes every point in the flow paths in every breathing cycle.
European Application 164 500 describes another type of anaesthetic machine. This known anaesthetic machine has a fan to which a first system of flow paths and a second system of flow paths are connected. The first system includes an inspiratory line, a positive pressure reservoir, an inspiratory valve, an expiratory line, a negative pressure reservoir and an expiratory valve. The second system includes a return line between the fan's inlet and outlet and a pressure-controlled valve. A predetermined pressure gradient can be generated by the fan and maintained between the inspiratory section and the expiratory section of the first system. By regulating the inspiratory valve and expiratory valve, the breathing of a patient connected to the known anaesthetic machine can be controlled. If pressure in the expiratory section drops too far, the pressure-controlled valve opens, enabling gas to flow from the inspiratory section to the expiratory section via the second system.
This known anaesthetic machine has a number of disadvantages. For example, it does not allow the patient to breathe spontaneously, since breathing is completely controlled by the inspiratory and expiratory valves. Moreover, this known anaesthetic machine cannot provide all the various kinds of controlled breathing modes. As a result, this known anaesthetic machine has a limited range of use in practice.
European Application 813 883 describes a ventilator, such as one intended for respiratory care in the patient's home. In one embodiment, the ventilator has a gas reservoir with an inlet open to ambient atmosphere, a compressor for generating a flow of gas, a flow splitter that conducts gas, via an inspiratory tube, to a patient or, via a return tube, back to the gas reservoir. Oxygen can be dispensed into the return tube to raise the level of oxygen in the breathing gas.
This known ventilator, however, is not suitable for anaesthesia, mainly because it is open to atmosphere.